1. Field of the Invention
This invention relates to an apparatus for multiplexing a specific signal with an amplitude modulated television signal, transmitting and receiving the multiplexed signal, and extracting the specific signal from the multiplexed signal.
2. Description of the Prior Art
More than 30 years have passed since the color television broadcasting of the current NTSC (National Television System Committee) system began in 1960. Recently, in search of finer definition and higher performance television system, several new systems including HDTV (High Definition Television) system developed by NHK (The Japan broadcasting Corporation) have been proposed. At the same time, the contents of the programs presented to viewers have been changed from the mere studio programs to programs providing higher quality images and more realistic feeling such as cinema-size movies.
The current NTSC broadcasting is standardized by 2:1 interlaced 525 scanning lines, luminance signal bandwidth of 4.2 MHz, and aspect ratio of 4:3. (See, for example, Pritchard, "US Color Television Fundamentals --A Review", IEEE Trans. Consumer Electron., vol. CE-23, pp. 467-478, Nov. 1977.)
In this background, several television signal composition methods aiming at compatibility with the current broadcasting system and enhancement of horizontal resolution have been proposed. One of such examples is presented in a paper of Fukinuki and Hirano, "Extended Definition TV Fully Compatible with existing Standards", IEEE Trans. Commun., vol. COM-32, pp. 948-953, Aug. 1984. Considering the NTSC television signal expressed on a two-dimensional plane of temporal frequency f1 and vertical frequency f2, chrominance signals C are present in the second and fourth quadrants due to the phase relations to the chrominance subcarrier fsc. The Fukinuki et al example uses the vacant first and third quadrants for multiplexing the high frequency components of luminance signal. The chrominance signal and the multiplex high frequency components are separated and reproduced at the receiving end, thereby enhancing the horizontal resolution. In this example, the conventional NTSC receiver would be interfered by the multiplex signal, because the example has no ability for separating the chrominance signal from the multiplex high frequency components.
In the current television broadcast, as clear from the description above, signal bandwidth is limited by the standard, and it is not easy to add some new information in good quality. For example, other methods to enhance the horizontal resolution are proposed (M. Isnrdi et al, "A Single Channel NTSC Compatible Widescreen EDTV System", HDTV Colloquium in Ottawa, Oct., 1987), but many problems are left unsolved from the viewpoint of the compatibility with the current television broadcasting and deterioration of demodulation characteristics of high frequency components in a moving picture. Besides, from the standpoint of effective use of the frequency resources, the transmission band cannot be extended as an easy way.
We invented a method of superposing a signal by using quadrature modulation of the video carrier (U.S. patent application No. 70804) By this method, various signals can be transmitted using newly established quadrature channel and the interference to the conventional NTSC receiver is very small in principle. But the interference can be detected in practice, because of the incompleteness of characteristics of filters at the receiver and transmitter.
This invention is one of solutions to this practical problem when quadrature modulation is carried into practical use. Even if the incompleteness of such circuits as filters occurs, the interference to the conventional NTSC receivers can be reduced down to the acceptable level. In this sense, this invention is very useful when quadrature modulation of the video carrier is implemented.